Fluid density distribution in a high temperature, stratified thermohaline system: implications for saline hydrothermal circulation

EPSL ELSEVIER

Earth and Planetary

Science Letters 146 (1997) 121- 136

Fluid density distribution in a high temperature, stratified thermohaline system: implications for saline hydrothermal circulation Alan E. Williams

*

Department of Earth Sciences and Institute of Geophysics and Planetary Physics. University of California. Riverside. CA 92521, USA Received

16 May 1996; revised 15 October 1996; accepted

17 October

1996

Abstract Density distribution within the Salton Sea geothermal system, of fluids ranging from 20°C to 325X, has been computed using chemical and thermal data from geothermal production well tests and curve-fit models of Na-Ca-K chloride solution properties. Density corrections can easily be made to iO.01 g/cm3 for solute effects of each of the dominant chloride salts as well as pressure above vapor saturation. Field data on dissolved CO, is too sparse to routinely compute a correction, however this moderately, but variably abundant component can be shown to produce only minor ( < 0.01 g/cm”) errors in density estimates. Fluid density within the shallow, cool ( < 25O“C), low salinity portions of the system decreases markedly with increasing depth and temperature, from = 1.0 to = 0.85 g/cm3. A sharp interface separates these relatively dilute fluids from hypersaline brines with TDS > 20 wt%. The density of brine climbs rapidly to near 1.0 g/cm’ as the salinity increase across this interface overwhelms the thermal effect on fluid density. This steep density gradient precludes all but diffusional-conductive or perhaps double diffusive-convective mass and heat transfer in this transitional regime. Measurement uncertainties for reservoir depths, temperatures and salinities commonly exceed errors of our density model, limiting the accuracy of details in our modeled density distribution. Gross scale relationships of this salinity stratified geothermal system are, however apparent, permitting rational discussion of extrapolations to conditions beneath the presently explored reservoir and inference of dynamics during the systems’ evolution. Keyvords: density; stratification:

convection;

brines; Salton Sea geothermal

1. Introduction Since discovery of high salinity thermal brines [l] in what became known as the Salton Sea geothermal

* Tel: + 1 909 787 4611. Fax: E-mail: williams@ucracl .ucr.edu 0012-821X/97/$12.00 Copyright PII SOOl2-821X(96)00206-3

+ I 909 787 4509 or 4324.

field

system (SSGS, Fig. I), much interest has been expressed not only in the unusual chemistry of such fluids, but also in their circulation dynamics. After more than three decades of extensive scientific investigation and exploitation for geothermal power, the SSGS is undoubtedly the best explored high-temperature example of heat and mass transfer in a natural thermohaline setting.

0 1997 Elsevier Science B.V. All rights reserved.

122

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Science Letters 146 (1997) 12/L136

duce high salinities in active magma-hydrothermal systems such as in Mid-Ocean ridge settings [4-61, the Red Sea rift [7], at Larderello, Italy [8] and deep beneath the Geysers, USA [9]. Similar processes apparently permitted trapping of fluid inclusions with widely varying salinity in these environments’ fossil equivalents: ophiolites [lo,1 I] and epithermal and porphyry copper ore deposits ([12,2,13] and references therein). Mississippi Valley type ore deposits [ 14,2] also illustrate the importance of advective flow of large quantities of high salinity, moderate temperature, perhaps connate or evaporitic fluids. Salinity stratification and density-related effects on heat and mass transfer are crucial to our understanding of these geologically common and economically important environments.

1. I _ Previous work in SSGS

Fig. 1. Map of the SSGS, indicating locations of geothermal wells (0). rhyolite volcanic domes (stippled), rivers (thin solid lines) and roads (thin dashed lines) near the southeastern shore of the Salton sea (shaded). Concentric regions (heavy dashed curves) of the thermal dome are indicated.

A variety of hypothetical heat/mass transfer models, ranging from free convection to density stratified static, have been inferred from SSGS data, typically in studies involving detailed investigation of limited data sets. This work computes the density profiles in the SSGS from a wide array of data on measured reservoir temperature, pressure and chemical composition of produced fluids. The density profiles show an abrupt increase at the boundaries of a saline dome at l-2 km depth. Density inversions clearly indicate a strong driving force for convection above the dome and probably indicate convection within the dome. Fluid inclusion and geochemical evidence of extremely variable and high salinities in many other active and fossil hydrothermal settings [2,3] imply that thermohaline circulation plays important physical and chemical roles in such environments. Phase separation concentration of solutes and injection of magmatic volatiles are commonly invoked to pro-

Due to high temperatures and high heat flow observed in early investigations of the SSGS area, White [ 151 as well as Deutcher et al. [16] suggested that vertical heat transport was provided by largescale convective fluid flow throughout the permeable reservoir. This relatively simplistic, well-mixed reservoir model was abandoned when large compositional inhomogeneities were identified, requiring isolation of fluids in different regions of the reservoir. After exploratory drilling of the SSGS, Helgeson [ 171 detailed the somewhat perplexing hydrologic and thermodynamic state of this geothermal resource. Based on the limited array of wells available, Helgeson [ 171 recognized clear salinity variability, both with increasing depth and with location in the field. His thermochemical evaluation of the SSGS reservoir identified depth vs. temperature, fluid pressure and salinity gradients which appeared to preclude free convection [17]. The SSGS illustrated a linear hydrostatic pressure gradient of 0.0295 atm/ft (9.81 X lo-’ bar/m), consistent with fluid density near 1.0 g/cm-‘. It was therefore proposed [17] that in the SSGS, salinity varied systematically with temperature, to maintain unit density in hydrostatic equilibrium with the dilute, cold waters of the surrounding basin. To provide adequate heat flow in a generally non-convective system, [17] developed a model involving multiple cells of localized free convection.

A.E. Williams/Earth

and Planetan

with horizontal isolation permitting salinity differences between adjacent wells. A complex convective model was also developed by Younker et al. [18], involving an impermeable mudstone cap and interbedded reservoir which produced vertically separated convective cells. Heat transfer between cells was presumed, due to conduction across shale interbeds, while extensive heat and mass transfer to the boundaries of the field were accomplished by horizontal flow away from the Salton Sea (Fig. 1). This lateral flow hypothesis was of an further developed 1191, with the inclusion isolated heat source beneath the rhyolite domes (Fig. 1). The impermeable cap-rock layer, utilized in previous models to limit mass and heat flux to the surface, was first disputed by Rex [20,21]. His model, with density stability as the limiting upward boundary of the reservoir, proposed hot, saline fluids stably overlain by a dynamically separate, low salinity groundwater. In addition, Rex 120,211 postulated an extensive, deep, saline “brine pool” as the primary source of upwelled geothermal fluids. In this model [20,21], the primary hypersaline reservoir of the SSGS is in non-convective density balance produced by dissolved salt osmotic and thermo-osmotic effects. A more detailed description of thermal and salinity gradients necessary to produce a vertically non-advetting reservoir was provided by Michels [22]. Double-diffusive convection [23,24], proposed for the SSGS by Foumier [25,26], is a mechanism providing high heat flow in the presence of unit density fluids and non-convective thermohaline gradients. Because of the opposing effects of salinity and temperature, this diffusional mechanism gradually develops small, stacked convective cells. Each separately convecting cell becomes isolated from adjacent cells by a dynamically stable interface across which temperature and mass can be transferred only by conductive-diffusive mechanisms. Evidence of the geometry of a fluid interface [27-291 in the SSGS has been developed from physical, chemical and petrologic data ([29] and references therein). This fluid interface appears to be a major feature of the SSGS reservoir, sharply separating shallow, dilute fluids (typically < 10 wt% TDS) from deep, hot (> 26O”C), hypersaline brines ( > 20 wt% TDS) of the production geothermal reservoir.

Science Letters 146 f 1997) 121-136

123

The present work seeks to quantify across this brine interface.

2. Thermo-chemical voir

conditions

density profiles

in the SSGS reser-

2. I. Temperature Temperatures and thermal gradients [ 17,301 are highly variable across the explored SSGS reservoir. Temperatures increase monotonically, but with variable gradients (Fig. 2). Near the center of the field (Fig. 11, steep gradients (Fig. 2a) produce temperatures which reach 300°C at depths of _ 1.0 km. Below this depth, gradients decrease (Fig. 2a) but temperatures climb at least to 360°C in the deepest wells [ 17,301. In the surrounding “boundary” region (Fig. I), gradients are somewhat depressed (Fig. 2b) in comparison to wells in the central region. Temperatures here also increase to at least 355°C [31] at the 3.2 km depth of the Salton Sea Scientific Drilling Project (SSSDP) well State 2-14. More nearly linear thermal gradients are typical of the outermost, “distal” (Figs. 1 and 2c) geothermal wells of the SSGS, and gradients in deep non-geothermal regions (Fig. 2a) of the Salton Trough [32]. The SSGS geothermal anomaly appears as an elongate dome [29], with thermal contours roughly paralleling the field divisions illustrated (Fig. 1). Thermal contours do not follow major structures or stratigraphy [29] but the elongation of the dome (Fig. 11 does appear to correspond roughly to a string of rhyolite volcanic domes. Despite the high salinity, high-temperature SSGS fluids typically show good agreement between measured reservoir temperatures and those computed using the Na-K-Ca geothermometer [33] used here for most density computations. Below 200°C. this geothermometer consistently overestimates reservoir temperature, even when Mg corrected, providing anomalous values of 150-200°C for cold surface, lake and groundwaters. Temperatures measured, computed from production data or estimated from thermal gradients have been used for density computations in such intervals (Tables l-3). Use of lower, estimated values provides a conseruatiueZy high density computation for cool, dilute intervals.

A.E. Williams/Earth and Planeta? Science Letters I46 (1997) 121-136

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neous flow from dilute and saline intervals within the same well [27,29]. In all regions of the SSGS, salinities show an abrupt transition (I 0.5 km thick) from dilute to hypersaline fluids with increasing depth (Fig. 3). Production fluid data [29], indicate that the sharp salinity interface follows the 260°C isothermal surface, rather than any structural or

(b)

41 CC) 100

200

300

400

TEMPERATURE (C) Fig. 2. (a) Thermal gradients (data sources referenced in text) of wells typical of the central SSGS region. Typical non-geothermal Salton Trough gradient shown for reference. (b) Thermal gradients of wells typical of the boundary SSGS region. (c) Thermal gradients of wells typical of the distal SSGS region.

c*l 8

B

g2

E3 c3 4

2.2. Salinity stratification The salinity of fluids from the SSGS (Fig. 3) indicates a bimodal distribution [29], with dilute (typically < 10 wt% TDS) and hypersaline (> 20 wt% TDS) end-members. Few produced fluids have intermediate compositions, and these can often be shown to be artificial mixtures produced by simulta-

0

4

8

12

16

20

24

28

SALINITY (wt % TDS) Fig. 3. (a) Salinity gradients (after [29]) typical of the central SSGS region. Rectangles illustrate depth ranges of fluid production intervals. Non-geothermal Salton Trough gradient (after [32]) shown for reference. (b) Salinity gradients (after [29]) typical of the boundary SSGS region. (c) Salinity gradients (after [29]) typical of the distal SSGS region.

A.E. Williams/Earth

Table 1 Central region: representative

measurements,

estimates

and Planeta?

Science Letters

and computations

of physical and chemical conditions

Sample Eolall SS-nbr MP-rr Pion 1ID-3 CW-j2 Mm-Id3 wo-l-l CW-j5 El-l-l Mm-l CW-llm cw-I 1s wo-1 Mm-ldl CW-ut cw-14 CW-6 cw-12 cw-10 cw-13 cw-11 CW-dl cw-9 cw-7 CW-j 1 CW-e2 CW-j6 IIDCW-e14 CW-e13 CW-e10 El-l-2 IID- I sp- 1

0.44 0.26 2.15 0.58 1.70 0.65 2.15 2.91 3.30 3.77 3.70 3.00 3.35 3.03 3.80 3.58 4.94 4.61 4.61 4.45 4.60 4.84 4.73 4.42 4.09 4.83 4.75 5.90 5.78 5.38 5.75 5.80 5.89 5.50

0.48 0.26 1.00 0.48 1.20 0.42 1.80 1.63 2.10 2.20 2.20 1.90 2.09 1.70 2.30 2.18 2.67 2.72 2.52 2.64 2.56 2.15 2.83 2.53 2.25 2.74 2.76 3.11 3.18 3.14 3.10 3.04 2.95 2.94

Duplicate or similar samples represented by “typical” computation, others Na-K-Ca geothermometer.

kolall 0.02 0.01 0.45 0.03 0.22 0.08 0.31 0.46 0.55 0.55 0.6 1 0.49 0.50 0.45 0.53 0.57 0.75 0.73 0.71 0.70 0.12 0.73 0.75 0.70 0.65 0.74 0.75 0.94 0.88 0.86 0.84 0.87 0.94 0.83

0.01 0.0 1 0.01 0.03 0.06 0.04 0.15 0.17 0.22 0.24 0.27 0.26 0.28 0.25 0.32 0.32 0.39 0.40 0.39 0.41 0.41 0.43 0.44 0.4 I 0.38 0.48 0.50 0.57 0.60 0.59 0.59 0.59 0.60 0.59

values. # = mixed fluid; * = estimated

stratigraphic feature. SSGS fluid inclusion data (1291 and references therein) supports both the abrupt nature of this fluid interface and its coincidence with the 260°C isotherm. This apparent, sharp transition has been ascribed [ 171 to drawdown of dilute, cooler fluids into significantly warmer production intervals. The consistent nature of numerous dilute, shallow production intervals throughout the SSGS (Fig. 3), the similar salinity vs. temperature pattern of production fluid and inclusion data [29] and the improbability of short

125

146 (1997) 121-136

in the SSGS

Depth

Density

(m)

(g/cm’)

0 0

450 520 600 590 680 450 600 690 850 750 1050 800 1200 1100 840 1100 900 1100 900 900 900 840 1000 1000 1000 1140 1200 1200 1500 1450 1500 1400 or measured

1.006 * 0.996 * 1.036 * 0.921 ’ 0.941 0.861 0.948 0.941 # 0.953 # 0.969 # 0.959 # 0.918 * 0.937 # 0.915 # 0.953 0.939 0.994 0.98 I 0.977 0.969 0.972 0.982 0.977 0.963 0.949 0.97 1 0.965 1.009 1.001 0.986 1.003 1.003 1.004 0.988 temperature

used for density

flow tests consistently drawing groundwaters through the impermeable “cap” proposed by [17,18] all argue against such an interpretation. 2.3. Fluid chemistry Numerous studies of SSGS fluids ([17,30,34,29] and references therein) confirm that all but the most dilute are dominated by dissolved chloride salts of sodium, calcium and potassium [20,21,29] (Tables 1-3). Ratios of these major cations in SSGS thermal

126 Table 2 Boundary

A.E. Williams/Earrh

and Planetary Science Letters I46 (1997) 121-136

region (see Table 1)

Sample

MP-et MP-wi MP-dv MP-spa MSpa Mco2 cw-113 CW-b-l cw-112 Si-3-l cw-112 CW-b-2 cw-112 cw-I 12 Si-4 Si-3 CW-d4 CW-d8 CW-d3 CW-d2 CW-d6 St- 1 CW-e6 CW-r3 cw-e4 CW-r7 RR- 1 St-214-3 St-214-l St-214-4 St-214-2 Hu-lb

Density

Eolal)

Zolal)

K (molal)

Depth

Eolal)

(m)

(g/cm’)

0.83 0.24 0.68 0.35 0.36 0.39 0.23 0.93 1.70 0.94 2.42 1.49 3.21 3.42 5.83 4.79 4.26 4.38 4.98 4.67 4.92 4.58 5.24 5.53 4.74 5.44 6.00 5.79 5.80 5.81 6.06 5.82

0.62 0.29 0.49 0.31 0.32 0.32 0.23 0.70 1.15 0.67 1.54 0.95 1.93 2.02 3.46 2.86 2.48 2.45 2.78 2.62 2.80 2.66 2.79 2.95 2.69 2.98 3.04 3.18 3.08 3.16 3.25 3.00

0.06 0.02 0.05 0.02 0.02 0.03 0.00 0.07 0.16 0.10 0.29 0.18 0.45 0.47 0.89 0.74 0.66 0.65 0.77 0.68 0.73 0.68 0.81 0.87 0.74 0.81 0.97 0.93 0.91 0.88 0.97 1.12

0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.06 0.12 0.08 0.18 0.13 0.27 0.29 0.49 0.43 0.39 0.39 0.43 0.42 0.46 0.46 0.50 0.53 0.50 0.55 0.57 0.59 0.57 0.60 0.62 0.62

0 0 0 0 0 230 990 900 1000 1200

1.026 0.995 1.018 1.001 l.ooo 0.997 0.903 0.872 0.885 0.843 0.909 0.860 0.927 0.934 1.031 0.992 0.965 0.97 1 0.994 0.977 0.986 0.967 0.989 1.015 0.966 1.008 1.022 1.020 1.012 1.018 1.026 1.006

fluids [29] change regularly with increasing temperature and TDS, indicating a “sodium depletion” [20,29] of the brines by reaction with the host sediments. Various dissolved anions (BY, SO;*, HS and HCO;) and neutral species (H,CO,, H,SiO,, B(OH), and H,S) are also found in significant quantities in SSGS fluids. Dissolved carbon dioxide, however, is the only component observed to reach concentrations large enough (2 1 wt%) to influence the physical properties of the fluid phase. Scientifically and economically interesting quantities of base metals (Fe, Mn, Zn and Pb), minor alkali and alkaline earth elements (Sr, Ba and Li) and precious metals (Ag, Au and Pt) are also found in the high

temperature, 35,291.

I100 1200 1100 1100 1500 1800 1400 1500 1400 1200 1400 1400 1300 2600 1250 2500 2000 2500 1850 2500 2500 1800

hypersaline

3. Density computation

reservoir

* *

* * * *

# # #

brines

[ 1,15,28,

methods

Simple linear and quadratic curve fits were performed on NaCl, CaCl, and KC1 solution density databases [36-391 in order to develop empirical equations-of-state that permit reasonably accurate estimation of geothermal fluid density under reservoir conditions. Because drilling, production, thermal, physical and chemical data from SSGS wells are of variable quality, and are often incomplete, simplified

A.E. Williams/Earth

and Planetary

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Science Letters 146 (1997) 121-136

Table 3 Distal region (see Table 1) Sample

De-l-d De-2 La-3 De-I-d1 KF- 1-d La-2 La-2-d KF- 1 De-l La- I Bt-3 Fe-l Fe-6- 1

0.18 0.23 0.29 0.15 0.10 1.02 0.36 0.69 0.17 1.07 2.50 5.58 5.16

Fe-5 Fe-6-2

5.89 5.55

Na (molal)

Ca (molal)

K (molal)

kPh (m)

Density

0.23 0.31 0.40 0.17 0.08 0.90 0.37 0.57 0.19 0.94

0.00 0.00 0.00 0.00 0.00 0.05 0.00 0.02 0.00 0.06 0.34 I .03 0.84 0.9 I 0.97

0.00 0.01 0.01 0.00 0.04 0.03 0.01 0.09 0.01 0.06 0.17 0.48 0.52 0.56 0.57

1000 1550 1400 1520 1630 2290 1820 2500 2440 2300 3ooo 2700 3000 2750 3000

0.974 0.971 0.958 0.945 0.930 0.940 0.902 0.909 0.877 0.912 0.943 1.032 1.005 1.029 1.017

1.58 3.10 2.83 3.10 3.00

models for computing approximate fluid densities were used to avoid over-analysis of often poorly constrained conditions. Computations accurate to within kO.01 g/cm3 were utilized for estimation of NaCl solution density as well as all corrections used to compensate for chemical and physical variations in the reservoir. Major solute compositions, depths and computed densities are summarized (Tables l-31, for the central, boundary and distal regions (Fig. I> of the SSGS. Representative computations were used for intervals where numerous individual analyses indicate consistent values. For intervals where few analyses exist or where significant variability is observed, complete arrays of data were computed to represent fully ranges of physical and chemical parameters and uncertainties in this database of variable quality. The Na-K-Ca geothermometer [33] was used for T > 200°C intervals, while either measured or estimated temperatures were used elsewhere. Occasionally, necessary parameters were estimated, based on all available information [29], in order to provide complete, internally consistent approximations of reservoir conditions in situations involving missing or seriously anomalous data, analyses uncorrected for steam loss or wells where drilling/production history is unknown. For simplicity, a single depth has been used for computations

(g/cm31 * * * * *

*

#

(Tables l-3) although wells often produce from a wide depth interval (Fig. 3; depth ranges for intervals tabulated in [29]). Where possible, drilling, production or petrologic data constrain the depth used in density computations. In other cases, maximum or minimum depths have been chosen to provide conservativ& neutral (near to 1.0 g/cm31 density estimates in dilute and hypersaline intervals. 3.1. NaCl solution density For the dominant salt (NaCl), quadratic leastsquares fits were computed for concentration (Mcr) effects, from 0.5 to 7.0 molal, on the temperature dependent parameters (T = lOO-400°C) given by [37]. The resulting equation appears adequate for reproducing the vapor-saturated solution density tables of [37] to within the greater of either their reported error, or fO.O1 g/cm3 (Fig. 4a): pNaCltaf = A + BT + CT2

(1)

where A = 0.980 + 5.22 x lo-‘Mc,

- 2.22 x 10-3M;,l

B = - 1.71 x 1O-4 + 2.63 x 10-4Mc1 -

2.40 x lo- 5M,,2

A.E. Williams/Earth and Planetary Science Letters I46 (1997) 121-136

128

was required to fit the high temperature (300-400°C) and high salinity (4-10 molal) data of [39] accurately (Fig. 4a). With this combination of empirical relationships, density of dilute to 10 molal NaCl solutions at lOO-400°C and steam saturation pressures can be estimated to within 0.01 g/cm3 (Fig. 4a).

0.02 0

-0.02 1

o.)02ii

3.2. Density correction for CaCl,

3 -0.q, , ,

(

,

,

,

,

,

,

,

1

(c) KC1 correction

o o2

(d) 100 bar pressure correction

I

In order to correct computed NaCl solution density for the presence of significant quantities of other cations, density difference ( Ap) relationships must also be fit to simple equations. For this study, ApCa_Na has been derived from the CaCI, database of [38] and NaCl database of [37]. Steam saturation pressure database differences were linear leastsquares fit for both temperature (loo-300°C) and Cl- concentration (1.0-7.0 molal) to provide an equation that reproduces pure CaCl, solution densities [38], to better than +O.Ol g/cm3 (Fig. 4b): Ap,,_,,

-0.02 100

150

= -5.00

X 1O-3 + 4.98 x lo-‘T

+ 1.47 x lo-*M,, 200

250

300

350

- 2.22 x lo-‘TM,,

400

(3)

Temperature (0 C) Fig. 4. (a) Plot of NaCl density residuals of curve tits (see text) to saturation pressure data tables from [37] (circles) and [39] (squares). 0 = data uncertainties > 0.01 g/cm3. (b) Plot of CaCl, density correction residuals of curve fits (see text) to data tables from [38]. (c) Plot of KC1 density correction residuals of curve fits (see text) to data tables from [36]. (d) Plot of 100 bar compressibility correction residuals of fits (see text) to 200-100 bar pressure data tables from [37].

and

= - 1.21 x lo-* + 4.97 x 10-5T

+ 2.23 X 10-3M, + 3.00 X IO-%l,

+ 8.26 x lO+M;,. =

A similar linear least-squares fit was performed using the difference between the KC1 database of [36] and the NaCl database of [37] at steam saturation conditions to develop Ap,_,,. The potassium density correction equation reproduces pure KC1 solution density [36] to better than kO.01 g/cm3 over the lOO-400°C temperature range and concentrations from 0.5 to 4 molal (Fig. 4~): Ap,_,,

C = 3.54 x 1O-6 - 9.54 x lo-‘Mc,

PNaCl(b)

3.3. Density correction for KC1

(4)

PNaCl(a)

+(Mc, - 6)(5.0 x 1O-3 + 1.25 X 10-4(T - 300)) (2) At T 2 300°C and M > 6 molal, however, a linear correction (Fq. (2)) to the NaCl curve fit (Eq. (1))

3.4. Linear mixing model for complex solutions To correct geothermal fluid analyses for effects of mixed salt compositions, a linear mixing model

A.E. Williams/

([40,25] and D.E. Michels, was used: fhd

=

PNaCl

+

+

(2MCa/MCI)

W,/%)

3.5. Compressibility steam saturation

Earth and Planetary

pers. commun.,

1983)

* Ak,~Na

* AfkNa

correction for pressures

(5)

above

Computations limited to steam-saturation conditions (Eq. (l), Eq. (2), Eq. (31, Eq. (4), Eq. (5)), must additionally be corrected for fluid compressibility. Compressibility from 100 to 200 bar above saturation appears insensitive to NaCl concentration [37], so all databases from 0.5 to 6 molal and 75” to 300°C [37] were linearly regressed to provide a compressibility (Eq. (6)) which reproduces the density of 100-200 bar NaCl solutions 1371, to well within +O.Ol g/cm3 (Fig. 4d). Accurate hydrostatic pressure measurements are seldom available in active geothermal systems, so for SSGS data pressures were approximated using the linear, near-unit density gradient observed by 1171 and supported by this study. Vapor pressures were approximated using a curve fit (D.E. Michels, pers. commun., 1983) of equivalent NaCl solution vapor pressures given by 1411:

ScienceLetters 146 (19971 121-136 3.7. Density corrections for dissotued CO,

Carbon dioxide is a variable, but often important component of geothermal fluids. In the SSGS, concentrations ranging from 500 to > 10,000 ppm have been reported ([ 17,34,30,29] and references therein) with highest values in cool, dilute portions of the field. Unfortunately. data on this component in SSGS thermal fluids is sparse, thus specific correction for any density effects is difficult. Computations illustrating the range of density corrections appropriate in SSGS fluids were therefore performed using the method of [42,43]. Over the temperature range from 300” to 550°C XNaC, from 0.0158 (5 wt%) to 0.0923 (25 wt%) and X co1 IO.0041 (10,000 ppm), it can be shown that the effect of CO, will be to lower the fluid density by I 0.01 g/cm3. This effect is greatest for CO,-rich, saline fluids which are rare in the SSGS [ 17,34,30] ([29] and references therein). Not specifically correcting for such effects provides a conservatively high density for fluids, particularly in cool, dilute portions of the reservoir where CO, is abundant.

4. Results and discussion

4.1. Density-temperature APNac,/bar

129

relationships

= (2.50 X lop5 + 2.99 X lO~‘T)/lOO (6)

3.6. Density corrections for other dissolced metais Although geothermal fluids are chemically complex, high salinity brines are dominated by Na, Ca and K chloride salts. Salton Sea geothermal brines contain as much as 3000 ppm of dissolved metals (predominantly Fe, Mn, Zn and Pb [29]), but even these concentrations are small in comparison to the three dominant cations (total N 90,000 ppm). Since additional cationic components are minor, density effects of other metal chlorides have been approximated by equivalent amounts of NaCl. This is easily achieved by computation of pNaC, using Mc, rather than M,. (e.g., Eqs. (1) and (2)).

Fluid density computations clearly indicate that the SSGS, as a whole, does not involve simple unit-density covariance of temperature and salt concentration (Tables 1-3, Fig. 5). Within the dilute, cooler layer of the system ( < 26O”C), density regularly declines with increasing temperature, reaching values as low as 0.85 g/cm3. Since temperatures monotonically increase within the explored portion of the reservoir (Fig. 21, this computed inverse density gradient (Fig. 5) implies a strong driving force for convective overturn. At temperatures greater than 260°C (Fig. 5), hypersaline fluids routinely occupy the reservoir and computed fluid densities rapidly climb to near 1.0 g/cm3. The variable quality of available temperature, depth and/or fluid chemistry data probably produce much of the scatter observed.

A.E. Williams/ Earth and Planetav Science Letters 146 (1997) 121-136

130

Mixing due to turbulence, diffusion or double-diffusive convective processes all will act to disperse this sharp discontinuity as the system evolves. Unfortunately, due to the typically poor constraints on production depth (Fig. 3), it is impossible to quantify the absolute thickness of this intermediate, interface zone. It is, however, certainly narrower than 500 m (Fig. 3) and covers a temperature range less than 30°C (Fig. 5). 0.8;

1 0

1 100

1

1 1 1 1 1 200 300 400 Temperature (C)

1

1 500

Fig. 5. Fluid density computations for SSGS dilute fluids (01, well-bore mixtures (01, hypersaline fluids ( n ) and extrapolations to high Cl- concentrations and temperatures (curves). Fields represent extrapolations for isosaline (convection) and isochoric (thermohaline balance) conditions at depths below the explored SSGS.

4.2. Fluid inter$ace Dilute and hypersaline segments of this densitytemperature trend are connected by data representing intermediate, mixed fluids (open symbols, Fig. 5). Although these are plotted as single points, most represent wells which produce drastically variable waters. Such variations have been interpreted as indications of simultaneous production of chemically differing fluids entering the well bore at different depths [27,29]. Multiple producing intervals are common in wells of commercial geothermal fields since broad open intervals (depth ranges on Fig. 3) are used to insure adequate fluid production capacity. Where well-bore fluid mixing has been observed (open symbols, Fig. 5) density computations should not be interpreted as representative of reservoir conditions. In fact, other data (solid symbols) must be interpreted with caution since some may also represent well-bore mixtures not revealed due to limited study under variable fluid production conditions. Regardless of the nature of intermediate fluids, it is apparent that either a steep positive density-temperature gradient or discontinuity (Fig. 5) exists between dilute and hypersaline fluids of the SSGS. The steep and positive nature of this offset, which represents a density difference of + 0.05 to + 0.15 g/cm3, guarantees that this interval will act as a stable, non-convective interface separating the drastically dissimilar fluids.

4.3. Density-depth

relationships

The distributions of both temperature and salinity within the SSGS have been shown to form an elongate, domal shape [30,28,29] that can be divided into regions (Fig. 1) having similar thermal (Fig. 2) and salinity (Fig. 3) gradients. When computed fluid density is plotted against depth (Fig. 6), it becomes apparent that the complex density profile, including the fluid interface (positive density gradient) is an extensive, continuous feature which grades to progressively greater depths away from the center of the thermal anomaly. Computations thus support the dome-shaped “brine diapir” model proposed by [28] and qualitatively described by [29]. The sinuous density vs. depth curves (Figs. 5 and 6) indicate three separate and distinct dynamic regimes, arrayed as a layered domal structure (Fig. 7). The shallowest of these regimes illustrates an unstable, inverted density distribution, perhaps driving convective circulation in dilute fluids which exist to depths of about 0.5, 1 and 2 km in the central, boundary and distal regions of the SSGS, respectively (Fig. 6). A poorly constrained, but probably narrow depth interval of strongly increasing fluid density (Fig. 6) universally underlies the dilute, convective regime. This interface acts as a stable, non-convective boundary between dissimilar water masses above and below. Data from the deepest regime, particularly that from the boundary region (Fig. 6b), however, makes it clear that fluids do not continue to increase in density as a stable, stratified reservoir [21]. Instead, density appears (Figs. 5 and 6) either to become constant at near unit-density [ 17,22,25,26] providing a stable, perhaps diffusive-convective environment,

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determine

the dynamic state of this deep, unexplored conclusively. Interestingly, the relatively consistent chemical and isotopic compositions [29] of deep, high-temperature fluids produced by SSGS wells (Fig. 3) imply relative homogeneity and perhaps mixing within the hypersaline reservoir. Density computations from hypersaline wells in the central portion of the thermal dome (2 1.00 g/cm3; Fig. 6a) indicate slightly lower values than those in either boundary or distal regions (1 .O- 1.03 g/cm’; Fig. 6b,c). This regional variation provides additional support for a convective deep reservoir, since it may indicate upwelling (Fig. 7) beneath the central dome, but outflow and subsidence of boundary and distal fluids beneath the interface (Fig. 7). Similar radial density gradients are expected in the brine diapir model [28,29], when advecting fluids become more dense as they conductively transfer heat across the interface to the shallow convective regime. reservoir

b)

4.4. Pressure

gradients

expected ,for density stratifi-

cation

.85

.90 DENSITY

.95

1.00

1.05

@n/cc)

Fig. 6. (a) Fluid density gradient for central SSGS region indicated by computations from discrete interval fluid data (0) and mixed interval data (0). Approximate non-geothermal gradient illustrated for reference. (b) Fluid density gradient for boundary SSGS region indicated by computations from reservoir fluid data (symbols as in (a)). (c) Fluid density gradient for distal SSGS region indicated by computations from reservoir fluid data (symbols as in (a)).

or begins to decrease slightly [29], permitting convective overturn and mixing deep within the reservoir. As observed by [ 17,22,25,26,29], uncertainties in the production depth, temperature and chemistry of presently available data make it impossible to

Upon cursory examination, it appears difficult to justify the constant unit-density pressure profile observed by [17] with the severe density variations computed for the SSGS (Fig. 6). Static pressure gradients in production geothermal wells are notoriously difficult to resolve by field measurements [22,29] since the presence of long intervals of either open hole or annular space commonly provide average pressures rather than that of one specific zone in the undisturbed reservoir. The high quality data of [17] should, however, be considered as a test for any model of density distribution in the SSGS environment. Computations of cumulative hydrostatic pressures (Fig. S), provided by summing pressure contributions of 100 m segments of columns of stratified fluid (Fig. 6) illustrate that actual effects of complex density distributions on the observed pressure gradient will be minor. In fact, density gradients computed for the SSGS (Fig. 6) particularly that from the central region, do not conflict signzjkantly (Fig. 8 inset) with the central region pressure data on which [ 171 developed the unit-density model.

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CONTOURS

1

2

0

surface

6

4

ISOCHORIC

2

CONTOURS

4

6

DISTANCE FROM THERMAL DOME (km) Fig. 7. Schematic cross-section, with no vertical exageration, across the SSGS thermal stratification (hypersaline = heavy stippling, interface = light stippling, dilute = no stippling) _. and potential circulation

(arrowsj

dome. Thermal contouring (“C) and salinity shown on left. Fluid density contours (g/cm’)

shown 0; right.

4.5. Extrapolations to unexplored depths and temperatures

0

low

zoo0

DEPTH

3ooo

4coo

(meters)

Fig. 8. Pressure gradients computed for central (solid line). ary (short dashed line) and distal (short-long dashed line) using fluid density gradients (Fig. 6a-c, respectively). Oval expanded to compare stratified density profiles (lines) with SSGS pressure data (symbols) from [17].

boundSSGS, region central

Computations of saline fluid density to conditions outside of those in the presently explored SSGS can be performed using the empirical curve-fit relationships discussed, if we assume that hydrostatic pressures can be maintained. It is, however, quite likely that hydraulic interconnectivity will be limited at rather modest depths and temperatures (370-400°C) by the brittle-plastic transition within silicic sediments [441. Chemical compositions of fluids for extrapolations can be estimated from the SSGS Na-K-Ca trends [29] and the Na-K-Ca geothermometer [33], which can be reasonably assumed valid to temperatures where metamorphic reactions eliminate minerals controlling solute activity ratios [33,441. Using these assumptions for extrapolated temperature (350-475°C) and depth (4-6 km) conditions, estimated densities of Cl- isopleths have been illustrated (Fig. 5). As indicated by the field of “ thermomust haline balance” (Fig. 5), salt concentrations increase from approximately 6 molal in observed SSGS brines, to about 8 molal at 400°C if an inverse (convective) density gradient is to be avoided. Solutions having on the order of 10 molal Cl- are apparently required to prevent gravitational destabilization and the introduction of convection when

A.E. Williams/Earth and Planetary Science Letters 146 (19971 121-136

temperatures reach 450°C. Constant fluid compositions will produce densities which decrease strongly with increasing temperature (“convection”, Fig. 51. Despite the scatter of density computations from this variably constrained set of data, the trend of values for the most highly saline fluids (near the 6 molal Cl- isopleth, Fig. 5) appears to elongate along an iso-saline (convective) orientation rather than along one indicating thermohaline stability. Although temperature gradients (Fig. 21 in deep portions of the reservoir may be low, nearly iso-saline conditions could easily produce an inverse density gradient capable of convective overturn. 4.6. Constraints on origin and evolution of stratified geothermal

systems

In the SSGS, salinity (Fig. 3) and fluid density (Figs. 5 and 6) gradients imply a drastic, and relatively sharp fluid interface, which serves to dynamically separate two distinct, only loosely coupled regimes of fluid circulation. Since most convective, diffusive and dynamic mixing processes tend to decrease rather than accentuate compositional gradients, creation and maintenance of such a discontinuity is difficult [20,21]. The origin of the interface and initial stratification most likely date to the early history of the SSGS. Given the geologic setting of the SSGS ([29] and references therein), early formation of a deep and extensive “brine pool” 120,211 drastically more saline than overlying basin waters is most easily accomplished prior to development of a thermal anomaly. Such a feature may have been produced by local evaporative brine production in the ancient lake Cahuilla ([20,21,29] and references therein) and gravitational sinking of the brines, which pooled along the base of the permeable sedimentary section [20,211, perhaps at depths of 4-5 km. Although detailed thermal and mass transport modeling will be needed to combine shallow heat flow data with the concept of stratified subsurface fluids presented here, advective behavior in the SSGS can be qualitatively described. Upwelling both of the brines and their isolating interface to the present domal configuration (Fig. 71 can be accomplished by thermally induced advection 128,291. Heating of the base of the system and rapid energy transfer of convective or double-diffusive mechanisms within

133

the deepest regime (Figs. 6 and 7) will act to warm and, thus further advect, the density-stabilized interface layer as a unit. A reduction in heat flux from the base of the system can similarly cause the interface layer to cool and descend as a unit, providing opportunity for retrograde mineralization as observed in the SSGS by [28]. This behavior of wholesale advection of the fluid interface has been described as “brine diapirism” [28,29]. It is expected that similar systems could evolve in any geologic environment where abrupt salinity stratification develops prior to introduction of a local heat source. Continental rift systems such as the Salton trough or the East African rift, are ideal environments, due to the development of isolated, deep, sediment-filled basins which often include lacustrine/evaporite salt concentrations. Such rifts also localize intrusion and volcanism, which serve as local heat sources driving hydrothermal circulation. In other geological settings, salinity stratification can be provided by mechanisms which operate during, rather than prior to, thermal activity. Solute concentration by boiling, evolution of deep, saline igneous or metamorphic waters and dissolution of evaporitic salts into already convecting thermal fluids all provide salinity inhomogeneities which may produce thermohaline stratification. The dynamic nature of these environments, however, make it less likely that a regionally extensive, and compositionally sharp interface could survive to evolve into a distinct brine diapir. This is particularly true for systems where salinity increase is thermally induced or is dominated by phase separation processes. Simultaneous introduction of thermally driven dynamics and fluid salinity variations make it difficult to produce a discrete and extensive density-stabilized interface capable of evolving into a “diapiric” structure. Appropriate heat and mass transport simulations will be necessary to constrain dynamics in hydrothermal systems involving complex, thermal- and salinity-influenced density variations.

5. Summary Fluid densities computed for the Salton Sea geothermal system (SSGS) range from somewhat more than 1.0 g/cm3 to less than 0.85 g/cm”.

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Regionally consistent temperature- and salinity-related variations in density far exceed errors from uncertainties in data or the rudimentary curve-fit model of Na-Ca-K chloride salt solution properties used. Despite radical local variations in density, computed pressure gradients agree surprisingly well with observed unit density hydrostatic conditions. The SSGS apparently does not conform to previously proposed uniform density models but is, in fact, a complex, density-stratified geothermal system. Depth profiles consistently indicate three distinct layers which illustrate differing dynamic behaviors. Fluid density within the shallowest, cool and low salinity layer of the system decreases markedly with increasing depth and temperature, from = 1.0 to = 0.85 g/cm3. This inverted density gradient is gravitationally unstable and should permit large-scale convective circulation. A sharp interfacial layer apparently separates shallow, relatively dilute fluids from deep hypersaline brines. This interface is localized along the 260°C isothermal surface rather than by any structural or sedimentological features. Most intermediate salinity samples are mixtures of discrete dilute and saline reservoir end-member fluids from different depths within the broad, open intervals of most geothermal wells. Although production depth uncertainties limit resolution of this interface region, fluid densities climb rapidly with increasing depth and temperature, to 2 1.0 g/cm3 as the effect of increasing salinity overwhelms the thermal effect on density. This steep positive density gradient provides gravitational stability and will preclude all but diffusional-conductive or perhaps double diffusive-convective mass and heat transfer in this transitional regime. The deepest, hot, hypersaline layer indicates a relative stabilization of reservoir fluid density at values near 1.O g/cm3. Uncertainties in often poorly constrained depth, temperature and salinity data prevent definitive differentiation between convective and diffusive-convective models for the deep SSGS. Lower densities near the center of the geothermal system and slight decreases in density with increasing temperature and depth appear to favor models involving deep convective heat and mass transfer. The evolution of this density stratified thermohaline system is most easily described by two separate

stages: (1) Generation of hypersaline brines by evaporative means and pooling of dense fluids at the base of the permeable section, separated from dilute basin waters by a sharp salinity interface [20,21,29]. This probably occurred prior to significant thermal input. (2) Localized heating of deep, hypersaline brines and their bounding interface, producing wholesale advective rise in the form of a dome-shaped “brine diapir” [28,29]. Such behavior is reasonable in light of conductively limited heat transfer across the interface and the more rapid heat transfer within the convective upper and lowermost layers of the system.

Acknowledgements The author gratefully acknowledges necessary background support from NSF (EAR-8303557, EAR-8502405, EAR-8805426), California State University-wide Energy Research Group (UCB/ UERG/ Williams-88), DOE (DE-FG03-85ERl3408) and Battelle Project Management Division (Contract E5 12-08300). Assistance in obtaining access to SSGS wells, samples and unpublished information is also acknowledged to D.E. Michels, Republic Geothermal Inc., Magma Power Co., Dow Chemical USA, Kennecott Corp. and Bechtel International. This paper was improved greatly by the helpful reviews and comments of Dr. R.O. Foumier, Dr. L. Cathles and Dr. M. Kastner. [IVK_~

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